Memory for computers or other electronic devices can include blocks of memory cells integrated into a larger integrated circuit or stand-alone integrated circuits. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), static RAM (SRAM), synchronous dynamic RAM (SDRAM), flash memory and phase change memory. Phase change memory devices utilize materials that have different electrical properties in their crystalline and amorphous phases. Each phase change memory cell may be programmed by putting the material in the memory cell into either a crystalline phase or an amorphous phase providing non-volatile memory that does not require power to retain its contents. Phase change memories are often programmed using heat generated by an electrical current to control the state of the phase change material.
Phase change memory cells may be made from chalcogenide materials. Chalcogenide materials include at least one element from Group VI A of the periodic table. Chalcogenide phase change material, when heated to a temperature above its melting point and allowed to cool quickly, will remain in an amorphous glass-like state with a high electrical resistance. The chalcogenide phase change material, when heated to a temperature above its glass transition temperature Tg but below the melting point, will transform into a crystalline phase with a much lower resistance. This difference in the material properties between the amorphous and crystalline phases of chalcogenide materials may be used to create a phase change memory device.
In a phase change memory device, a high peak current is involved to program a phase change memory switch (PCMS) cross-point array. Either the driver devices should have a high current density so they can be built at the same pitch as the memory array via simple metal wire routing, or the driver devices otherwise should be large enough to provide sufficient current, in which case they are not the same pitch as the memory array and are placed off to the side which complicates metal wire routing, or the pitch of the array is relaxed to accommodate the larger driver devices.